Acrylic acid is generally produced by a two-step oxidation process which comprises catalytically reacting propylene with oxygen at high temperature in the presence of a catalyst to produce acrolein and then catalytically reacting the acrolein with oxygen.
In recent years, however, processes for producing acrylic acid from propane through one-step oxidation are being studied in order to reduce the cost of acrylic acid production. The main subject in these studies is to seek for catalysts which give acrylic acid in high yields. Oxidation reaction catalysts for obtaining acrylic acid from propane (hereinafter referred to as "catalysts for acrylic acid production") have been proposed.
Examples thereof include a Bi--Mo--V oxide (see U.S. Pat. No. 5,198,580), an Mo--V--Te oxide (see JP-A-7-10801 and JP-A-6-279351), an Mo--Sb--P oxide (see U.S. Pat. 4,260,822), a V--P--Te oxide (see JP-A-3-70445 and Catal. Today, Vol. 13, 679(1992)), and a Bi--Mo--V--Ag oxide (see JP-A-2-83348). (The term "JP-A" as used herein means an "unexamined published Japanese patent application".) Among the catalysts enumerated above, those with which acrylic acid is obtained in high yields are the Bi--Mo--V oxide and the Mo--V--Te oxide. In the references disclosing the former catalyst, there is a description to the effect that acrylic acid can be produced in a yield of 5% with a selectivity of about 28%. In the references disclosing the latter catalyst, there is a description to the effect that acrylic acid can be obtained in a yield of from 35 to 40% with a selectivity of from 55 to 60%.
In general, the performance of a catalyst does not solely depend on the kinds and proportions of the constituent metals, but considerably depends also on the valences and crystal structures of the constituent metals. It is generally known that the valences and crystal structures of constituent metals contained in catalysts vary depending on catalyst production processes.
The aforementioned Bi--Mo--V oxide catalyst or Mo--V--Te oxide catalyst for acrylic acid production is also produced by a technique intended to impart excellent catalytic performance. Specifically, the technique employed for producing the catalyst comprises evenly mixing telluric acid, ammonium paramolybdate, ammonium metavanadate, or bismuth triacetate, in a heated aqueous medium, evaporating the water to obtain a solid mixture, and calcining the solid mixture at 400 to 600.degree. C. A technique known to be effective in enhancing catalytic performance comprises mixing compounds of antimony, niobium, vanadium, and molybdenum in a heated aqueous medium to obtain a slurry and mixing this slurry with a separately prepared slurry containing a bismuth compound and a molybdenum compound (see U.S. Pat. No. 5,198,580).
On the other hand, catalysts produced from a vanadium compound/antimony compound mixture obtained through the reaction shown by the following scheme (1) are known as catalysts not for the production of acrylic acid, but for producing acrylonitrile by oxidizing propane in the presence of ammonia, i.e., as catalysts for the ammoxidation reaction of propane. Examples of these ammoxidation catalysts include a V--Sb metal oxide (see U.S. Pat. No. 5,498,588 and JP-A-2-180637) and an Mo--V--Sb metal oxide (see JP-A-9-157241). EQU V.sup.+5 +Sb.sup.+3 .fwdarw.V.sup.+3 +Sb.sup.+5 (1)
The above reaction is usually conducted using a compound of trivalent antimony, e.g., antimony trioxide, and a compound of pentavalent vanadium, e.g., ammonium metavanadate, in an aqueous medium at a temperature of 80.degree. C. or higher.
However, reaction (1) shown above has not hitherto been employed for producing a catalyst for acrylic acid production.